This invention relates to the continuous manufacture of shaped articles, such as siding and molding, by vacuum forming over a molded belt impression, and more particularly, to selective cooling and shaping procedures for producing building products, and the like.
Continuous and semi-continuous processes for creating patterns on extruded plastic sheets have been used in the building components industry for a number of years. Some prior systems have disclosed rigid linked patterns for forming shaped impressions in an extruded sheet material. Unfortunately, such rigid shaped patterns tend to form unsightly horizontal seams in the material. Other systems have used pattern forms on rotating cylindrical drums. Although these processes are continuous, and do not produce horizontal seams, they often require expensive additional instrumentation to align the arcuate surface of the pattern with the relatively flat surface of the product.
Because of the limitations on prior continuous processes, some manufacturers have opted for injection or blow molding such products one at a time. While this technique can provide the desired detail in texture and surface finish, it is limited to product sizes of about 4-5 feet in length and provide product thicknesses which are practically limited to greater than about 0.080 inches. This is generally because of the limitations associated with flowing hot polymer through thin cross-sectional profiles in steel molds. Additionally, because of the known size limitations, the randomness of individual features on the surface of the product is limited. This is because only a relatively small number of pattern elements, such as shingles, can be molded into the relatively small surface area. When several of these products are aligned side by side on a wall of a building, for example, it is sometimes obvious to see the pattern repeated over and over again.
One solution to these problems is disclosed in Bosler, U.S. Pat. No. 5,314,325 dated May 24, 1994. Bosler""s invention provides a continuous vacuum forming process which uses a resilient mold belt for providing semi-continuous production of almost unlimited lengths of shaped polymeric articles. He further teaches a single cooling step whereby the entire width of the product is brought below the heat deflection temperature of the polymeric material.
It has been now determined, however, that a single cooling step reduces the temperature of the edge portions below a comfortable range for subsequent sizing and forming. Accordingly, there remains a need for providing for the separate formation of edge portions, such as butt-edges and nail edges customarily used in the fabrication of siding panels.
The present invention provides both methods and apparatus for manufacturing shaped polymeric articles. In the first method, a sheet of extruded polymeric material is provided. The sheet is disposed onto a rotating belt which includes a mold impression and a plurality of apertures through the belt. A vacuum pressure is applied to the hot polymeric material through the belt so as to draw the extruded sheet into intimate forming contact with the mold impression. This forms a patterned sheet portion and a remaining sheet portion. The patterned sheet portion is cooled below a heat deflection temperature of the polymeric material, and then the remaining sheet portion is shaped and formed before it is also cooled to produce a continuous shaped sheet. The sheet can then be severed to produce a desired length for the product.
This invention can produce continuous lengths of siding and molding products with greater randomness and better visual aesthetics. The vacuum forming process can produce product with thicknesses of as little as 0.005 in., and preferably about 0.30-0.120 in. Because of the continuous nature of the process, its thin wall capability, and efficient use of factory floor area, the resulting cost per square foot of sheet material made pursuant to this invention can be substantially less than molded products.
The present invention improves upon known vacuum forming processes, such as disclosed in Bosler""s ""325 patent, by providing means for keeping the hot outer edges of the product above the heat deflection temperature of the polymeric composition while the central vacuum formed portion is cooled. This permits these edges to be later formed, for example, by subsequent sizing operations, to create desired detail, such as nail and butt edges.
The use of a flat mold belt enables the formation of a long product while using a small processing station. It has been discovered that the length of the product can actually be longer than the total circumferential length of the mold belt. That would mean that one portion of a continuous flexible pattern could be utilized in the forming of two or more portions of one piece of product. Thus a very long product could be vacuum formed utilizing a relatively short belt.